1. Field of the Invention
The present invention relates to an apparatus, a receiver and a method for timing recovery in an orthogonal frequency division multiplexing (OFDM) system, and particularly for sampling timing recovery implementation in a multiple input multiple output OFDM (MIMO-OFDM) system for wireless communications.
2. Description of Related Art
MIMO-OFDM technology is extensively employed in IEEE802.11n based high-speed Wireless Local Access Networks (WLAN), mobile systems with Worldwide Interoperability for Microwave Access (WiMAX), and Digital Video Broadcasting (DVB) systems. It is well known that OFDM is robust in frequency selective fading channels with a relatively low implementation cost and high bandwidth efficiency. Moreover, MIMO takes advantage of multipath propagation to increase throughput, range/coverage, and reliability. By transmitting multiple signals containing different information streams over the same frequency channel, MIMO provides a means of doubling or tripling spectral efficiency. The combination of MIMO and OFDM techniques provides low-cost implementation particularly for high data rate communications.
However, OFDM is known to be vulnerable to synchronization errors, which can cause inter-carrier interference (ICI) and degrade system performance. In MIMO-OFDM transmission systems, the synchronization tasks include carrier frequency synchronization and timing recovery, and timing recovery can be further divided into symbol synchronization and sampling clock synchronization. Symbol synchronization finds the correct position of the Fast Fourier Transform (FFT) window with the aid of the dedicated training symbols. The purpose of sampling clock synchronization is to align the sampling clock frequency of the receiver to that of the transmitter, that is, to compensate the sampling phase drift between the transmitter and the receiver.
There exist many publications dealing with timing recovery for OFDM or MIMO-OFDM receivers. Examples of these are:
[1]. Hlaing Minn, Vijay K. Bhargava, and Khaled Ben Letaief, “A Robust Timing and Frequency Synchronization for OFDM Systems”, IEEE Trans. On Wireless Comm., Vol. 2, No. 4, July 2003, pp. 822-839.
[2]. Michael Speth, Stefan A. Fechtel, Gunnar Pock, and Heinrich Meyr, “Optimum Receiver Design for Wireless Broadband Systems Using OFDM-Part I”, IEEE Trans. On Comm. Vol. 47, No. 11, November 1999, pp. 1668-1677.
[3]. Nguyen Duc Long and Hyuncheol Park, “Joint Fine Time Synchronization and Channel Estimation for MIMO-OFDM WLAN”, IEEE Proc. of Intelligent Signal Processing and Communication Systems, ISPACS 2004, 18-19, November 2004, pp. 463-467.
[4]. Seok Ho Won, Deuk-Su Lyu, and Hyeong Jun Park, “Physical Layer Implementation and Evaluation of Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing (MIMO-OFDM) system”, IEEE Proc. International Conference on Communication Technology, ICCT 2003, Vol. 2, Apr. 9-11, 2003, pp. 1348-1352.
[5]. Chi-Yeh Yu, Zih-Yin Ding, and Tzi-Dar Chiueh, “Design and Simulation of a MIMO OFDM Baseband Transceiver for High Throughput Wireless LAN′, IEEE Proc. of 2004 Asia-Pacific Conference on Circuit and Systems. Dec. 6-9, 2004, pp. 205-208.
[6]. Aliert van Zelst and Tim C. W. Schenk, “Implementation of a MIMO OFDM-Based Wireless LAN System”, IEEE Trans. on Signal processing, Vol. 52, No. 2, February 2004, pp. 483-494.
[7]. Shou-Yin Liu and Jong-Wha Chong, “A Study of Joint Tracking Algorithms of Carrier Frequency Offset and Sampling Clock Offset for OFDM-based WLANs,” IEEE Proc. of Communications, Circuits and Systems and West Sino Expositions, Vol. 1, Jun. 29-Jul. 1, 2002, pp. 109-113.
[8]. Baoguo Yang, Khaled Ben Letaief, Roger S. Cheng, and Zhigang Cao, “Timing Recovery for OFDM Transmission”, IEEE Journal of Selected Areas in Communications, Vol. 18, No. 11, November, 2000, pp. 2278-2291.
[9]. Pei-yun Tsai, Hsin-Yu Kang and Tzi-Dar Chiueh, “Joint Weighted Least Squares Estimation of Frequency and Timing Offset for OFDM Systems over Fading Channels”, IEEE Trans. on Vehicular Technology, Vol. 54, Issue 1, January 2005, pp. 211-223.
However, a majority of the work addressing the problems in quickly acquiring the varying sampling phase and stably tracking the timing drift with a low complexity actually focus on symbol synchronization [see for example the publications by Hlaing Minn, Vijay K. Bhargava, and Khaled Ben Letaief, entitled “A Robust Timing and Frequency Synchronization for OFDM Systems”, published as IEEE Trans. on Wireless Comm., Vol. 2, No. 4, July 2003, pp. 822-839, and Michael Speth, Stefan A. Fechtel, Gunnar Pock, and Heinrich Meyr, entitled “Optimum Receiver Design for Wireless Broadband Systems Using OFDM-Part 1”, published as IEEE Trans. on Comm. Vol. 47, No. 11, November 1999, pp. 1668-1677.
Many sampling frequency offset estimation algorithms jointly with carrier frequency offset estimation only address the timing error estimation instead of recovery [see for example publications [3] to [7] mentioned above], and few works lend themselves efficiently to integrated circuit implementations suitable for low cost WLAN products.
A non-coherent delay-locked loop based sampling clock recovery scheme is proposed in the publication by Baoguo Yang, Khaled Ben Letaief, Roger S. Cheng, and Zhigang Cao, entitled “Timing Recovery for OFDM Transmission”, published in the IEEE Journal of Selected Areas in Communications, Vol. 18, No. 11, November, 2000, pp. 2278-2291. The scheme described in this publication improves the mean square error performance compared to the commonly known correlation methods, and refers to tracking the timing drifting caused by the sampling frequency offsets. However, its acquisition is slow and tracking performance is poor particularly in low SNR and multipath fading channels due to its coarse phase discrimination algorithm.
A jointly weighted least squares estimation of carrier frequency offset and sampling frequency offset in OFDM systems is proposed in the publication by Pei-yun Tsai, Hsin-Yu Kang and Tzi-Dar Chiueh, entitled “Joint Weighted Least Squares Estimation of Frequency and Timing Offset for OFDM Systems over Fading Channels”, published in the IEEE Trans. on Vehicular Technology, Vol. 54, Issue 1, January 2005, pp. 211-223. Although this algorithm derives the optimal weight factors and can achieve near the Cramer-Rao low bound in the variance of estimation errors, its complexity is very high and it may not be suitable for practical application. Moreover, its accurate estimation is not integrated with an efficient PLL implementation, and, therefore, its acquisition and tracking performance are not promising due to the high dynamics in the estimator output or the loop filter input.
Thus, there is a need for a system and method which can ameliorate or substantially overcome the abovementioned disadvantages.